Page pointer
Take a look at the official instructions for set 40252.
An example of a Page pointer is seen at page 10 and point to the previously assembled submodel. LPub3D can do this too!
Download a prepared example LDraw file here to work with
Open this file in LPub3D and navigate to page 8
Right click the assembly image of step 6 and choose <Place Page Pointer>
In the dialog make sure Relative to is set to <Page> and position [Left]
Click [OK]
Click the black arrow (this is the page pointer) and manipulate the handle at the arrowhead similar to the pointers for callouts
Drag it until it points to the place where the submodel should go and repeat the dragging if necessary to put the arrowhead at the right position. This can be somewhat cumbersome.
Give the arrow a thin white border.
The result should look something like this:
